While processing semiconductor wafers it is often advantageous to deposit or form films that can later act as etch stop layers when subsequently deposited or formed films are removed. However, if the film does not have sufficient etch resistance during later processing, such films can be inadvertently removed.
One example of inadvertent removal involves thin silicon nitride sidewall (or offset) spacers for MOS transistors. Thin silicon nitride sidewall spacers are commonly used as an implant mask to provide a space between the lightly doped drain (LDD) implants into the semiconductor surface and the gate stack. A typical process flow has a first spacer layer that acts first as an offset spacer, then as an underlayer/etch-stop while additional films, such as disposable second sidewall spacer comprising SiGe, is deposited on top, used, which is then subsequently removed. In one process flow hot phosphoric acid (HPA) is used to remove the second sidewall spacer. However, even silicon nitride spacers formed from bis-tertiarybutylamino-silane (BTBAS) and ammonia reagents, where BTBAS-based silicon nitride is known to be the most wet etch resistant silicon nitride film to HPA, are not always capable of stopping the HPA etch when the disposable SiGe second sidewall spacer is removed. In particular, if the silicon nitride sidewall spacer has been exposed to reducing chemistries, such as plasmas containing H2 or N2, etch stop characteristics can be lost resulting in inadvertent removal of the silicon nitride offset sidewall spacer, and as a result subsequent shorting between the gate and source and/or drain, such as due to a subsequently deposited silicide ion the gate, source and drain. Moreover, as semiconductor devices are shrunk in size, and the distance between the top of the gate stack and the top surface of the source/drain regions is reduced, the probability of electrical shorts due to the silicide forming on the sidewalls of the gate stack increases.